This invention relates to a process in which the residual chloride impurities in crude alkoxysilanes are reduced in order to render the resultant alkoxysilane more suitable as a starting intermediate for the preparation of other chemical compounds and for use in electronics applications.
Alkoxysilanes are produced commercially via the reaction of chlorosilanes or organochlorosilanes with alcohols. One of the quality requirements for the subsequent use of alkoxysilanes as chemical intermediates in the preparation of other chemical compounds and for use in the electronics industry is the need for 5 low residual chloride content--i.e., levels down to 100 parts per million (ppm) chloride or lower on a weight basis.
Several process routes to alkoxysilanes are known in the art. Examples of known routes to alkoxysilanes are disclosed by Nitzsche et al., in U.S. Pat. No. 3,792,071, issued Feb. 12, 1974; Kotzsch et al., in U.S. Pat. No. 4,039,567, issued Aug. 2, 1977; Kotzsch et al., in U.S. Pat. No. 4,228,092, issued Oct. 14, 1980; and Schinabeck et al., in U.S. Pat. No. 4,298,753, issued Nov. 3, 1981. Another example of a process to produce alkoxysilanes from chlorosilanes and alcohols is a continuous process which comprises: (a) a distillation-column type reactor in which liquid reactants, intermediates, and product flow in a downward direction and in which vapor reactants, intermediates, and by-produced hydrogen chloride gas pass upward in countercurrent flow with liquids; (b) chlorosilane or organochlorosilane and a portion of the alcohol reactant are fed to the top of the column as liquid feeds; (c) the remainder of the alcohol reactant is fed in the lower portion of the reactor as vapor; (d) the product alkoxysilane is drawn off the bottom of the reactor as a liquid; (e) gaseous hydrogen chloride and vapors of reactants and intermediates pass out of the reactor to a means for contacting the gases and vapors with liquid methyl chloride, the methyl chloride vaporizing, cooling, and condensing vapors of reactants and intermediates, separating the gaseous hydrogen chloride as a gas with methyl chloride from the condensed reactants and intermediates; (f) passing the combined hydrogen chloride and methyl chloride stream to a means for separating and recovering the by-produced hydrogen chloride; and (g) returning the cooled liquid stream of reactants and intermediates to the reactor at a point below the entry of the feed. As a result of this configuration, the upper portion of the reactor is maintained at a temperature well below the boiling point of either of the reactants.
While the processes discussed, supra, are designed to essentially react all chlorosilane materials to alkoxysilanes, residual chloride materials remain--i.e., 500-1000 ppm chloride on a weight basis relative to the alkoxysilane produced. The residual chloride may be unreacted chlorosilanes or organic chloride material. The source of these organic chlorides may be alkyl chlorides from the direct process reaction to produce organochlorosilanes or the reaction of hydrogen chloride with olefinic materials that are impurities in the organochlorosilanes. Whatever the source of the chloride impurity, many applications of the alkoxysilanes, such as use as a chemical intermediate and for use in electronics applications, require that the residual chloride content be as low as possible.
As an example of the need for a very low chloride content, certain electronics applications require that the alkoxysilane materials have electroconductivity of approximately 5-10 micromho/centimeter (micromho/cm.). The correlation between electroconductivity value and residual chloride content is not entirely clear; however, electroconductivity goes down as chloride content goes down.
A further example of the need for very low chloride content is the use of alkoxysilanes as chemical intermediates. A specific example of the use of alkoxysilanes as chemical intermediates is the preparation of a chemical material that is prone to be highly colored. The ultimate use of this chemical material dictates the need for low color. It has been found that chloride content of the alkoxysilanes has a significant impact upon the final color of this chemical material. This relationship will be illustrated in the examples, infra.
Burzynski and Martin in Great Britain No. 1,115,052, published May 22, 1968, disclose a process in which impure alkoxysilanes are purified to "an essentially zero acid content" by distillation of the desired alkoxysilane from a mixture of the crude alkoxysilane and a reagent that converts acidic or potentially acidic species to non-acidic, non-volatile compounds. Burzynski and Martin report chloride levels of treated alkoxysilanes as low as 1 ppm. The claims of Burzynski and Martin are silent on whether or not distillation is carried out at atmospheric pressure. However, all the examples disclosed are at atmospheric pressure. Specific materials purified by this technique included methyltrimethoxysilane, with an atmospheric pressure boiling point of approximately 100.degree. C. The reagents disclosed by Burzynski and Martin are such materials as LiAlH.sub.4, sodium methoxide, aqueous sodium carbonate, aqueous sodium hydroxide, and alkali metal salts of weak organic acids. The analytical test disclosed by Burzynski and Martin consisted of the titration of residual chlorosilane material and reagent alcohol with potassium hydroxide. The inventors of the instant invention submit that Burzynski and Martin were only able to analyze for unreacted chlorosilane materials and did not detect organic chloride materials using such a method. The inventors of the instant invention further submit that Burzynski and Martin neither recognized the presence of organic chloride nor reduced the organic chloride content of methyltrimethoxysilane or alkoxysilanes with an atmospheric boiling point of less than approximately 130.degree. C. as does the instant invention. In Example 12 of Burzynski and Martin methyltriethoxysilane with a chloride content, determined by the technique supra, of 20 ppm was distilled in the absence of any treating agent. The result was a distillate that analyzed by their analytical technique to contain approximately 150 ppm chloride. The inventors of the instant invention submit that in Example 12 of Burzynski and Martin organic chloride was present in the methyltriethoxysilane sample but was not detected by their analytical test. Upon heating, the organic chloride decomposed to form hydrogen chloride which then could be detected by their test. The conclusion one must draw is that Burzynski and Martin were not aware of the presence of chloride impurities other than ionic or hydrolyzable chloride materials. Burzynski and Martin disclose and claim a process for the removal of ionic chloride from alkoxysilanes with boiling points below approximately 130.degree. C. There is no teaching regarding the removal of residual chlorides, as taught by the instant invention.
Asano et al., in Japanese Patent Publication OPI No. 47931/75, published Apr. 28, 1975, discloses the purification of triethoxysilane by means of reflux and distillation and the use of an inert gas purge. The principal impurities of concern are unreacted ethanol, other ethoxysilanes, hydrocarbons, ethyl ether, and ethyl chloride. Ethyl chloride is the only organic chloride specifically mentioned. This method claims the reduction of impurities to levels lower than those attained with conventional distillation. However, one drawback to this method that is apparent to the inventors is the potential loss of usable product with the inert gas. Many of the noted impurities have boiling points close to that of the product triethoxysilane. As such, one expects that the inert gas bubbled through the crude triethoxysilane carries a significant amount of the product triethoxysilane from the distillation vessel. No mention is made of this potential problem, means to recover triethoxysilane from this gas stream, or for that matter, the overall recovery efficiency of triethoxysilane.
Bezlyudnyi et al., in Soviet Union No. 852,874, issued Aug. 7, 1981, disclose the purification of alkoxysilane compounds by treatment with calcium hypochlorite at temperatures of 100.degree.-140.degree. C. Bezlyudnyi et al., intends to oxidize crotonic aldehydes which appear to impart color to alkoxysilanes. No mention is made of the removal of chloride material from alkoxysilanes.
Chung and Hayes, J. Organometallic Chemistry, 265(1984), pp. 135-139, discloses a process in which the chloride contamination of methoxysilanes could be significantly reduced by refluxing the methoxysilanes with metallic sodium. Chung and Hayes disclose that the problem with chloride removal is the inability to remove the alkyl chlorides. Chung and Hayes disclose the use of three separate analytical tests to determine ionic chloride, residual chloride and acidity following hydrolysis of the methoxysilane. Chung and Hayes used the residual chloride test as the criteria for measuring chloride reduction. Refluxing of crude methyltrimethoxysilane with metallic sodium was found to reduce the residual chloride content of the methyltrimethoxysilane from 400 to 5 ppm chloride. Chung and Hayes also studied refluxing methyltrimethoxysilane with other reagents and were unsuccessful in significantly reducing residual chloride content. The other reagents studied were LiAlH.sub.4, tetramethylguanidine, sodium methoxide in methanol solution, potassium hydroxide in methanol solution, (C.sub.4 H.sub.9)SnH.sub.2, and magnesium oxide. The reagents studied by Chung and Hayes, with the exception of sodium metal, were ineffective in reducing the non-ionic chloride level of the crude methyltrimethoxysilane. These results are summarized in Table 1 of the reference of Chung and Hayes. Thus, Chung and Hayes teach away from the disclosure of Burzynski and Martin, supra. Further, Chung and Hayes teach away from the instant invention as will be apparent from an understanding of the invention taught herein. A disadvantage of the disclosure of Chung and Hayes is the issue of safety in the handling and processing of metallic sodium on an industrial scale.
Sugihara et al., in Japanese patent application No. 255469/85, filed Nov. 14, 1985, disclose a process in which the non-hydrolyzable chloride content, from sources such as carbon-bonded chlorine, of alkoxysilanes is lowered. Sugihara et al., disclose a process in which crude alkoxysilanes are (a) treated with an acid-treated clay or metal halide at the reflux temperature of the alkoxysilane, (b) treated with a neutralizing agent, and (c) filtered or distilled to remove the solids from neutralization. Sugihara et al., disclose that their process does not achieve low residual chloride levels with treatment with a neutralizing agent without the heat treatment of the crude alkoxysilanes with an acid-treated clay or a metal halide. Sugihara et al., thus teach away from the instant invention.
The objective of the instant invention is to provide a process that will safely and effectively lower the residual chloride content of crude alkoxysilanes. A further objective of the instant invention is to provide alkoxysilanes of suitable quality for use as a chemical intermediate or for use in electronics applications.
It was found that alkaline metal oxides and hydroxides such as magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, and zinc oxide are ineffective in reducing the residual chloride content of lower-boiling alkoxysilanes such as methyltrimethoxysilane when the metal oxide or hydroxide was contacted with the alkoxysilane under refluxing conditions at the atmospheric boiling point of these alkoxysilanes. However, it was unexpectedly found that by using pressure to elevate the boiling point of the lower-boiling alkoxysilane to temperatures greater than approximately 130.degree. C. in the presence of an alkaline metal oxide or hydroxide the residual chloride content of the crude alkoxysilane could be significantly reduced. By "significantly reduced" for the purposes of this invention, we mean that the residual chlorides in the alkoxysilane can be reduced greater than one-half of their original level in the alkoxysilane. More generally, the residual chlorides can be reduced to ten percent (10%) or less of their original level in the alkoxysilane by use of this inventive method.